O.S. Engines 25LA - Manual (Page 22)

18 Needle-valve adjustment(Summary)
Practical best(optimum) needle-valve setting
Exhaust note starts to change.
Disconnect battery leads from
glowplug at about this point.
Clear, high-pitched two-stroke
exhaust note
Maximum rpm setting("Lean").
"Rich" needle-valve
setting when starting
the engine.
The engine may stop if the battery leads
are disconnected from the glowplug while
the engine is running rich.
Note: Although this is a two-stroke engine it fires like
a four-stroke at these rich needle-valve settings-i.e.
ignition of the fuel charge takes place at every fourth
stroke of the piston instead of at every second
stroke.
Engine
stops
R
e
v
o
lu
ti
o
n
s
s
ta
rt
t
to
d
e
c
re
a
s
e
.
to
d
e
20~30˚
Intermittent, high-pitched
two-stroke note superimposed
on low "four-stroke" sound.
NOTE: The above sketch is for reference purposes only.
Actual needle positions may differ from those shown.
On starting from cold, with the needle-valve
set at the rich starting position:
Take note of this position of the needle-valve.
This will produce the practical best (i.e. optimum)
rpm setting (lower than maximum rpm).
A light grey exhaust emission may be observed.
Now, re-open needle-valve 20-30˚
maximum rpm is reached and will fall off (or engine will
stop) if needle -valve is closed any further.
Exhaust gas will be very light.
Finally:
exhaust note is now a steady high-pitched sound,rising
higher in pitch as needle-valve is closed and rpm increases.
The grey smoke will be lighter.
(However, make sure that engine is fully run in.)
Further needle-valve closure:
an intermittent high-pitched two-stroke note will be superimposed
on the low-pitched "four-stroke" note.
Exhaust smoke will be less dense and grey in color.
As needle-valve is closed and rpm increases
a good deal of white smoke is emitted, accompanied by
a relatively low-pitched "four-stroke" exhaust note.
6 7
1
2
5
4
3
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